Leptin inhibits glycogen synthase kinase-3β to prevent tau phosphorylation in neuronal cells

We have previously demonstrated that Leptin reduces extracellular amyloid β (Aβ) protein both in vitro and in vivo, and intracellular tau phosphorylation in vitro. Further, we have shown that these effects are dependent on activation of AMP-activated protein kinase (AMPK) in vitro. Herein, we investigated downstream effectors of AMPK signaling directly linked to tau phosphorylation. One such target, of relevance to Alzheimer's disease (AD), may be GSK-3β, which has been shown to be inactivated by Leptin. We therefore dissected the role of GSK-3β in mediating Leptin's ability to reduce tau phosphorylation in neuronal cells. Our data suggest that Leptin regulates tau phosphorylation through a pathway involving both AMPK and GSK-3β. This was based on the following: Leptin and the cell-permeable AMPK activator, 5-aminoimidazole-4-carboxyamide ribonucleoside (AICAR), reduced tau phosphorylation at AD-relevant sites similarly to the GSK-3β inhibitor, lithium chloride (LiCl). Further, this reduction of tau phosphorylation was mimicked by the downregulation of GSK-3β, achieved using siRNA technology and antagonized by the ectopic overexpression of GSK-3β. These studies provide further insight into Leptin's mechanism of action in suppressing AD-related pathways.